This invention relates to a unique combination of secondary refining processes external to the steelmaking furnaces such as the electric-arc, the BOF o the Q-BOP, for producing steels having any or all the combinations of the following elements in very low concentrations: Carbon content at less than 0.01% for most grades and at less than 0.0025% for some grades; phosphorous content at less than 0.010% for most grades and at less than 0.002% for some grades; sulfur content at less than 0.010% for most grades and at less than 0.002% for some grades; nitrogen content at less than 0.010% for most grades and at less than 0.0025% for some grades; hydrogen content at less than 0.0001% for most grades; and aluminum killed for most grades and less than 0.005% aluminum content for some grades.
The secondary refining processes can be performed in ladles, special vessels or molds and include vacuum degassing methods (vacuum ladle-to-ladle stream degassing, vacuum-oxygen refining, and vacuum ladle degassing), which by proper sequencing can produce the aforementioned grades of steel with restricted low chemical composition limits.
The inventors recognize that the ladle refining processes (vacuum ladle-to-ladle stream degassing, vacuum-oxygen refining, and vacuum ladle degassing) selected for producing the aforementioned grades of steel are presently being practiced separately but to their knowledge the processes have not been combined or properly sequenced to produce the aformentioned steel chemistries.
The presence of furnace slag in secondary or ladle refining processes has been recognized as a deterrent by those who are proficient in the art. Furnace slags may contain varying amounts of compounds of phosphorous, sulfur, iron, silicon, and alkalies which will contaminate the steel and/or erode the refractory linings in the ladles during the refining processes.
Several methods are used to separate the furnace slag from the steel in preparation for subsequent ladle refining processes. Although any of the positive methods for separating the furnace slag from the steel for subsequent ladle refining processes would be acceptable for the production of the aforementioned special steels, the two selected by the inventors are reladling and slag skimming. If the reladling process is selected (and it is preferred by the inventors), it should be performed by the vacuum ladle-to-ladle stream degassing method. The steel for this process should not be aluminum killed prior to reladling thereby permitting the removal of carbon, nitrogen, hydrogen, and part of the oxygen from the steel during the reladling operation. Also, as described in the embodiment of the patent, it may be necessary to use the slag skimming process for the grades of steel that require low (&lt;0.005%) aluminum contents in addition to the very low residual concentrations of the other elements.
For many grades of stee where low (&lt;0.01%) carbon contents are not required, it is common practice to use the ladle electric-arc reheating process for further refining the steel and for obtaining the desired steel temperature. However, for the grades of steel described herewith, ladle arc reheat process would not be acceptable because of the pickup of carbon from the graphite electrodes during arc reheating. To reheat the steel and to obtain the desired steel temperature the inventors employ chemical reheating by the addition of oxidizable elements, such as aluminum, to the steel and selective oxidation of the element by gaseous oxygen blown onto the steel within an enclosure where the absolute pressure in the enclosure is less than one atmosphere.
For the special grades of steel that require very low phosphorus and nitrogen contents it may be necessary to remove part or most of these undesirable elements by practices prior to the processes included in the embodiment of the invention.
The purposes for selecting the various ladle refining processes and their sequence of operations become apparent in the description of the invention.